Uninterruptible power supplies, which are typically abbreviated by UPS, take over power supply of different kinds of electric system in case of failure of a power mains supply, which provides power to the electric system in normal operation. Accordingly, the UPS comprises an AC power source, which can provide power to the electric system in case of failure of the power mains supply or the AC power grid. The failure of the power mains supply can be a complete shutdown, or merely a partial failure, e.g. when only a power level below the power required for the electric system can be provided.
The UPS typically comprises a system controller, which activates and controls the supply of power from an alternate AC power source to the electric system. The system controller is typically DC driven and powered from the AC power source, e.g. via a rectifier. This DC power supply is also referred to as DC bus.
A typical UPS includes one or more storage capacitors coupled to the DC link. Since large capacitors are typically used in the DC link, power-up of the UPS can result difficult. Some on-line UPSs use a boost rectifier, which allows the DC voltage on the DC link to be boosted to levels greater than the peak AC input voltage. However, a potential problem with using such a boost rectifier in combination with a large storage capacitor is that undesirably large inrush currents may be produced when the rectifier is coupled to an AC source such as a AC power gird. In particular, in a boost mode converter, the DC link storage capacitors may initially act as a short-circuited load to the utility through the boost converter.
A precharge system may be used to pre-charge the DC link to a voltage close to or preferably above the utility peak voltage at start-up. Common approaches include using a power resistor that is coupled to the DC link with or without a series control device, such as a relay, to precharge the DC link from the AC power supply. However, conventional precharge techniques provide a long and/or difficult to scale pre-charge time or, conversely, require the use of components that are bulky and expensive and may not be economical to assemble.
In order to enable a reliable power-up of the UPS in the case of an AC power absent environment, a dedicated power supply is used for the start-up phase, so that the UPS can be started also e.g. in the case of a blackout of the AC power grid without problems.